Antiprisms isomers

The two most important structines for [M(bidentate)s] are shown in Figure 16, drawn as idealized bicapped square antiprisms. Isomer I is possible at all values of the normalized bite whereas isomer II is expected only for larger chelate rings. A number of lanthanoid and actinoid complexes with four-membered nitrate or carbonate rings, [M (N03)5] and [M (C03)5] , have structure I. The only monomeric molecule in which structure II is observed is [Ba(MeCONHCOMe)5](C104)2 with six-membered chelate rings. 

The 48 vertices of the eight hexagons hnked by the Ai4,4 bipartite graph (Figure 1.16b) correspond to the 24 square antiprism isomers. 

From the graphs of (Ek/n) resulting from trajectories initiated from a distorted centered antiprism (isomer I) shown in Fig 2, it is possible to see that the fluctuations are present already at T = 182 K and become larger with increasing temperature. This is even more evident from the analysis in... 

There are extensive possibilities for the formation of geometric and optical isomers in eight-coordinate complexes. Thus far. apparently only one pair has been completely characterized The diglyme [= di(2-methoxyethyl)etherl adduct of samarium iodide. Sml rO(CH2CH2OCH3)2]2. has been isolated in both cis and irons forms. The trans complex (Fig. 12.39a) has a center of symmetry Thus, the I—Sm—1 angle is exactly 180. and the molecule is a bicapped trigonal antiprism. The cis isomer (Fig. (2.39b) has ihe lower symmetry of a distorted dodecahedron with I—Sm—I angles of 92 > ... 

Figure 86 The change in stereochemistry from a square antiprism to two interpenetrating pentagons as b is reduced for one of the possible isomers of [M(bidentate)3(unidentate)2l...

Figure 89 shows that at b = 1.2 an additional feature has appeared on the potential energy surface, which at b = 1.3 has formed a deep minimum (Figure 90). The two minima correspond to the two optical isomers of the Z)4 square antiprism (Figure 93). It may be noted that in contrast to the three-bladed propeller , which is the dominant stereochemistry for tris(bidentate) complexes (Section 2.3.3), this four-bladed propeller is only expected in tetrakis(bidentate) complexes where the bidentate ligands have exceptionally large normalized bites. At b = 1.26, the angle of twist 0 = 22.5° and the two square faces are staggered with respect to each other (Figure 93). In an analogous way to the behaviour of tris(bidentate) complexes, a decrease in b leads to a decrease in 9.

There are three important minima on the potential energy surfaces calculated for [M(bidentate)5]. At b x 1.1, isomers I and II correspond to two of the possible ways of arranging bidentate ligands around a bicapped square antiprism, whereas isomer III is a sphenocorona (Figure 102). 

By heating 2,4-( 2Br,H7 at 260° in the presence of C5H5Co(CO)2, two isomeric trimetallic complexes formulated as (CTDsCosC BsHv were isolated (76). One isomer has one Co-Co interaction with a high-coordinate carbon atom, whereas the proposed structure of the other isomer has the 3 cobalt atoms positioned on the same equatorial belt of the bicapped square antiprism, bound to each other, and the carbon atoms occupying the apical vertices. 

When bidentate ligands are involved in 8-coordinate complexes, many isomeric forms are possible for each polyhedron. The six isomers for a dodecahedron and the three isomers for a square antiprism are listed in Table 5.9. 

The deltahedron for n = 10, a bicapped square antiprism, exhibits two four-connect and eight five-connect vertices. Hence, for one heteroatom in a ten vertex c/oso-cluster we have 1 - and 2-isomers and two heteroatoms in 1,10-, 1,6-1,2-, 2,3-, 2,4-, 2,6- 2,8-isomers. Different placements generate different cluster stabilities. A rule of thumb is that the more electronegative element prefers the lower-connectivity vertex. Multiple heteroatoms more electronegative than B prefer non-adjacent positions as far apart as possible. Rearrangement to the most stable isomeric form need not be fast. In the case of icosahedral clusters, for example, the barrier to rearrangement is large and isomers can be isolated. 

Figure 2.6.6 The isomer ri -lSiyWICO) ], a bicapped square antiprism (the pseudo four-fold axis is vertical), where the transition metal is a part of the square prism and is five-coordinate with the cluster...

BTP = bicapped trigonal prism, DD = trigonal faced dodecahedron, SA = square antiprism. The isomer notation is taken from references168) and 169) and corresponds to the edges labelled in Fig. 2... 

The observed structure of [SngTl] is a bicapped square-antiprism. (a) Confirm that this is consistent with Wade s rules, (b) How many isomers (retaining the bicapped square-antiprism core) of [SngTl] are possible ... 

Both these papers describe the nse of qnantum chemical and molecnlar dynamics simulations to stndy the stractnre of the first and second coordination spheres of Th(IV) and to determine the residence time of water in the second coordination sphere. In the second paper the authors discuss the entry of chloride into the second coordination sphere at different concentrations of chloride. The starting point in these studies is the determination of the geometry and relative energy of different hydration isomers of Th(IV) nsing ab initio quantnm mechanics. When the ratio (H20)/(Th" ) is nine, the most stable stractnre, [ Th(H20)g" ] has a geometry (a capped square antiprism). 

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